Conventional lighting historically has used incandescent and fluorescent bulbs, but recently with the invention of the blue LED, has started to use LED lights. The initial cost of the LED light may be high, but over time the power savings can reduce the overall cost of lighting substantially. Part of the high initial cost of a power efficient LED light is due to the special electronics necessary to create a constant current to the LEDs from a power source. With this special electronics, however, implementation of features such as remote control, dimming, photo-sensing, timing, and color adjustment in the light are possible at very little additional cost. Such features for conventional lighting are performed by separate electronic units that turn power to the light on and off, which add cost and complexity.
Most LED lights today consist of multiple LEDs connected together in series and/or parallel, and are driven by a switching power supply. In an AC mains connected light, the power supply converts from the mains voltage, 85-240V, to a current for the LEDs, while the power supply in a battery powered light converts from the battery voltage to a current for the LEDs. Such circuits are offered by companies such as OnSemi and Supertex for mains connected, and Maxim for battery powered.
Less efficient LED lights simply connect the LEDs to the power supply through a series resistor. Although cheaper, the resistor dissipates substantial power, and when connected to an AC supply, the light has a poor power factor. The power factor is poor since the LEDs only conduct during the peaks in the AC waveform.
The LEDs in a light can be any color or any combination of colors, including white. White LEDs are typically made with a blue LED covered in some type of yellow phosphor. Much of the blue light from the LED is absorbed by the phosphor and re-emitted at lower frequencies corresponding to green, yellow, and some red colors. Some advantages of this approach include low cost and more natural continuous spectrum light. Some disadvantages include low efficiency due to losses in the phosphor, a bluish color from the LED, and reduced reliability due to degradation of the phosphor. Companies such as Cree Lighting and Nichia market such high brightness LEDs.
The spectrum of one particular Cree product shows a sharp peak around 450 nm, which is the blue light produced by the LED, and a broad peak around 550 to 600 nm, which is yellow from the phosphor. At 500 nm and 700 nm, the output power is only 20% of the peak power. In contrast, the spectrum of sunlight is virtually flat from just below 500 nm to just over 700 nm.
To overcome the lack of energy at the red end of the spectrum, Cree Lighting produces a two color overhead LED lamp that includes strings of red LEDs together with strings of phosphor coated blue LEDs. When comparing the spectrum produced by an RGB source, the Cree white LED plus red LED solution, and the standard output from an incandescent bulb, the spectrum of neither the RGB nor the white plus red light match either the incandescent or sunlight spectrum very well, although the white plus red light produces a good cost/performance compromise for many applications.
The ideal LED light from a color spectrum perspective would consist of many different colored LEDs operating at different power levels to produce a rough approximation of either incandescent or sun light. The combination of red, yellow, green, and blue is probably a minimum number of colors. Although this approach should have a good spectrum and be more energy efficient and reliable, control of the relative power levels in each color is difficult and expensive in practice today.
There are challenges to building even a three color (RGB) LED light that controls the color over process variations, temperature, aging, etc. Some techniques include feedback to the RGB driver circuits through three optically filtered photodiodes. Each photodiode is tuned to the color of each LED and is connected to a signal detection and signal processing function on an IC. The signal processor then controls the red, green, and blue drive currents accordingly. Such color filter photodiodes are offered by Hamamatsu, which are relatively expensive and consume board space that would otherwise be dedicated to producing light instead of receiving light.
National Semiconductor offers an RGB LED driver for LCD display backlighting. Their LP5520 can calibrate out the initial variation in LED optical output and then adjust over temperature. However, it does not compensate for aging. Since the output power of some LEDs goes up over time and some go down, the only effective means of compensation is through actual optical power measurement of each light component.
Cree's white plus red LED lamp includes two chains of 6 white LEDs, and one parallel/serial combination of 30 red LEDs, for a total of 36 LEDs. It also includes a photodetector and a temperature sensor to maintain color as described in US Published Patent Application No. 2008-0309255. A wavelength selective photo-detector monitors the shorter emission wavelength (green and shorter) and adjusts the brightness of the red LEDs in response. Likewise, temperature is monitored with a temperature sensing element, which is used to adjust the drive current to the red LEDs to compensate for brightness degradation with increasing temperature. Neither the optical power produced by the red LEDs nor the optical power produced by the white LEDs with wavelengths longer than green are measured. Any changes in red LED brightness over lifetime are not compensated. A cost effective solution for driving and controlling different color LEDs with good color discrimination and without the cost and board space for additional photodetectors and temperature sensors would be beneficial.
Conventional light dimming switches use a triac circuit that only allows the mains AC voltage to be applied to an incandescent light during part of the cycle. For instance, when set at half power, the voltage signal that passes through to the light is zero for the first 90 degrees of the sinusoidal voltage, jumps to the peak amplitude and follows the sinusoid down to zero for the second 90 degrees, stays at zero for the next 90 degrees, and finally jumps to the negative peak voltage and follows the sinusoid back to zero. This approach is a cheap and effective way for a consumer to dim a resistive incandescent bulb.
Although the triac dimmer reduces power consumption in the light bulb, it does not reduce the power that the utility company must produce. Power companies produce current that is in phase with the voltage. As the voltage increases, the current increases. If the entire load on a power generation plant consisted of lights dimmed 50% with triacs, the current produced during the first half of the positive and negative cycles would not go to the bulbs, but it would have to go somewhere. The utility must generate the same amount of power whether the lights are full on or dimmed and must deal with potentially dangerous transients on the grid.
The light from an LED can be reduced by either reducing the drive current or reducing the time that the current is applied by using what is called pulse width modulation (PWM). The current is turned on and off at a rate faster than the eye can see, with the duty cycle proportional to the desired light output. Since the wavelength of light produced by an LED changes with drive current, PWM dimming is sometimes preferred. When replacing an incandescent light with an LED light, an existing triac dimmer still adjusts the power supply to the light. To enable PWM dimming, the LED light circuitry must filter the power supply, detect the duty cycle of the supply, and adjust the PWM duty cycle accordingly, which adds cost and complexity.
Photosensors are commonly used to measure ambient light in a room or daylight outside and to adjust the brightness of a lamp in response. An outdoor lamp may be turned on and off at dusk and dawn respectively, or an indoor lamp may be dimmed such that the light from a window plus the light from the lamp remains constant. For existing technology, such photosensors need to be placed away from lamps so that the light from the lamp does not interfere with the photosensor. Typically, the photosensor is a separate electronic device, which needs to be installed. A lamp with a built in photosensor that is not affected by light output from the lamp and that does not require any wiring changes would be beneficial. Further, a lamp that could provide this functionality without a photosensor would be more beneficial.
Timers that turn a light on and off typically plug into a wall socket and connect and disconnect power to an attached light based on time of day. Such devices are typically bulky. An installed light socket cannot be changed to a timer without significant wiring changes. A replacement bulb that has a timer function built in that does not require additional cost or any wiring changes would be beneficial.
Electrical wiring and lighting switches in a new home or business construction consumes a significant portion of the construction cost. Additionally, light switches with dimmers are much more expensive than simple toggle switches, so therefore are used much less frequently. Lights that could be remotely controlled by for instance a device like a TV remote could significantly reduce wiring costs and provide additional features, which would be beneficial.
The developing world is leapfrogging developed world technology. For instance, solar powered homes are wide spread throughout the developing world (i.e., Kenya, India, etc.) Lighting traditionally has been provided by firewood and recently by kerosene, which are terribly inefficient. The combination of a solar panel, a car battery, and led lights provides a much better solution. During the day, the solar panel charges the battery and at night the led light consumes power. The effectiveness of the re-charging system determines the usefulness of the system. Any solar power efficiency improvement is significant. LEDs are photosensitive and can produce power when exposed to light. Harnessing this energy would be beneficial.
The invention described herein, in various embodiments, provides solutions to the issues described above.